Index   2015            2016            2017
1       NA              6355698         10107023
2       13000000        NA              NA
4       NA              NA              NA
5       NA              NA              8000000
6       331000000       659000000       1040000000
7       NA              NA              32032920
8       NA              NA              20000000
9       2521880         5061370         7044288
...

ind <- which(is.na(df), arr.ind=TRUE)
df[ind] <- rowMeans(df,  na.rm = TRUE)[ind[,1]]